drm/i915: vlv: fix RPS interrupt mask setting

This typo may lead to missed RPS interrupts and as a result a too
low or too high frequency for the current workload. The interrupt mask
will be set properly at a subsequent GPU idle event, but can get
corrupted again at the next RPS up/down event.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>